Editor’s Note: Miss the Pacific Northwest rain? It’s been 48 days (June 21st) since measurable precipitation at Tryon Creek State Natural Area. Enjoy this post about rainfall in the forest!
Article by Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
Mention “water” to anyone at Tryon Creek State Natural Area (TCSNA), and they will probably think of either the drinking fountain at the Nature Center, or Tryon Creek itself. However, we may need to consider other things in the park when someone brings up the topic of water. We can start by looking at the water cycle in the forest.
Here Comes the Rain
We are fortunate to be in an area with a pretty good rainfall. Sometimes it just drizzles, and sometimes it pours down. The first question is “where does the rain go?” Well, that depends on how heavy the rainfall is. This past April, I temporarily set up rain gauges at TCSNA when the forecast called for a rainy period for the next couple of days. I set up 2 rain gauges several feet apart under a large western hemlock (Tsuga heterophylla) and then placed a third rain gauge in a clearing less than 50 feet from the tree. I repeated this process with a large western redcedar (Thuja plicata). I checked the rain gauges after about a day of rain, and then again after 3 total days of rain. The results of both the redcedar and hemlock are combined and illustrated below:
It was astonishing to me that during the first 26 hours of rainfall totaling more than a third of an inch, that almost none of the rainfall penetrated the canopy of either tree. Okay, yeah, I know that when it starts to rain you head under a tree for shelter. But, I was surprised at how effective these under-tree shelters were. Even in the following two more days of rain, only a small portion of the water penetrated the canopy. For this three day event, only 18% of the total rainfall penetrated the canopy. No wonder there are few plants growing under mature trees of these two species.
I checked 2017 daily rainfall data collected by the City of Lake Oswego2 in downtown Lake Oswego, just a few miles from the park. The total annual rainfall was 53.13 inches. Based on my measurements during that one rain event, let’s assume that any daily rainfall of less than 0.35” will never hit the ground under these mature trees. In 2017, these light rains amounted to 25.9% of the total annual rainfall. Based on the information gathered in this study, none of that ever made it through the canopy. These means that plants growing under the canopy of redcedars and hemlocks experience a much different rainfall environment than other plants.
However, there can be lateral water movement in the soil once it hits the ground. To check that, I collected soil cores from beneath both the hemlock and the western redcedar. Under the redcedar the soil contained less water than in the surrounding areas beyond the redcedar’s canopy. For the hemlock, there was no difference between the under-the-canopy and outside-the-canopy soil water. This may have been due to the fact that the hemlock was growing on a significant slope, and the redcedar was growing in a flat area. Any rainfall uphill from the hemlock, probably traveled through the soil downhill to the hemlock.
And these aren’t the only species of plants that intercept the falling rain. Even our native Indian plum (Oemleria cerasiformis) seems to keep a lot of rain from ever hitting the ground, as seen in the picture below.
However, all is not lost. Numerous documents in the scientific literature point out that many plants can absorb water not just through their roots, but also through their leaves and needles.
An important function of the soil is to hold water for the plants to use. The forest at TCSNA is growing on soil that includes a significant layer of clay about 2-1/2 feet below the surface. Thus we see in some toppled over trees that the roots don’t go deep into the soil, but rather, tend to hit the clay layer and then begin to grow horizontally.
To determine how much water the soil holds, I used a soil corer to collect samples of only the top foot of soil at 21 locations at TCSNA. Thus this estimate of total water in the soil is VERY low, perhaps less than half of the water in the entire soil structure found at TCSNA. The approximate sampling locations are indicated on the map below.
I took the soil samples home and put them in plastic bowls to air dry. I weighed them periodically until they stopped losing weight. Then I calculated how much water was in the top 12 inches of soil at TCSNA. Then I carefully recalculated it 5 more times, because the answer astonished me. At the time I collected the soil samples, there was enough water in the top 12 inches of soil at TCSNA to fill 68 Olympic-sized swimming pools.
All plants need water to stay alive. As in humans, water is a key, and most often the dominant component of every plant. With the permission of TCSNA personnel, I collected the above ground parts of some plants, or parts of plants, and determined how much water they contained. The process was that I collected the plants in the forest, stuck them in a plastic bag, and immediately took them home and weighed them. Then I let them air dry in my garage. I periodically took the weights of each drying plant until the weight remained constant. Then I calculated the percent of water in the fresh plant. In a few cases the results were frankly surprising.
Latin Names not already noted: (Oregon grape, Mahonia nervosa; thimbleberry, Rubus parviflorus; swordfern, Polystichum munitum; horsetail, Equisetum sp.; red alder, Alnus rubra; English ivy, Hedera helix; waterleaf, Hydrophyllum tenuipes; jewelweed, Impatiens capensis;)
Plants contain a lot of water. Based on some samples I collected near the creek, if the entire park were covered in jewelweed about 4 feet tall (a typical mature height for this plant, the amount of water in the jewelweed would be more than enough to fill 1-1/4 Olympic sized swimming pools.
Both waterleaf and jewelweed will, under moist conditions, exude water from the edges of their leaves, especially on cool mornings. This is illustrated below (and no, it didn’t rain just before I took this picture).
The flip side of this is that waterleaf tends to wilt fairly easily on hot, dry days, as illustrated below.
In another spate of plant drying activity, I included the leaves of three species, and measured them on a schedule to compare how fast the leaves dried. The results are presented below.
The salal dried dramatically more slowly than either the elderberry or vine maple. This is not surprising because the salal leaves are much tougher than the other leaves. Salal is the only species of these three that holds its leaves over the winter.
It’s a wet, wet world
Water is unquestionably the dominant component of life on earth. The prominence of water in plants is documented above. Human beings, like me, and hopefully you, have been reported to contain somewhere between 55% and 60% water, with higher levels for infants. It is an amazing fluid that dissolves important nutrients, makes our cells turgid, and performs many other useful functions. Next time you see a rain cloud coming, be sure to step outside and say thanks.
1”Water, water everywhere,
And all the boards did shrink.
Water, water everywhere,
Nor any drop to drink.”
—- from The Ryme of the Ancient Mariner by Samuel Taylor Coleridge, 1797-1798
2 Thanks to Kevin McCaleb with the City of Lake Oswego for this data.
All photos by Bruce Rottink.
By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
The forest at Tryon Creek State Natural Area (TCSNA) contains marvelous plants that we can enjoy at different seasons for different reasons. They range from the beautiful trillium (Trillium ovatum) blossoms in the early spring to the bright red leaves of the vine maple (Acer circinatum) in the fall. But these individual displays of beauty are transitory, as are the plants themselves. This is summed up in the Latin title of this note which means: “thus passes the glory of the world.”1
I started a phenology study in mid-2013. This involved, for the most part, identifying and tagging specific individual plants and monitoring their developmental stages each year. These stages were things like when I could first see the veins on the new leaves, and the first time I found open flowers on the plant. Now, just five years later, I am surprised at how many of those individual plants I was following have died in that short timeframe.
Plants are Persistent
As we realize, plants are persistent. In the photo below, you see the result of a very old injury to the trunk of a Douglas-fir (Pseudotsuga menziesii) growing at TCSNA. Long ago, the upright shoot of this tree was damaged or killed, and several side branches competed to take over the role of “leader.” The branches marked with red arrows lost the race to become leader and are now dead. The branch indicated by the blue arrow won, and became the leader so successfully, that it looks almost like it was always the leader. Several branches that were lower down the tree when the top was lost are marked with green arrows, and they remained horizontal.
Another example of a persistent plant is this mature black cottonwood (Populus trichocarpa) located alongside the Old Main Trail. Normally, mature black cottonwoods don’t have little branches popping out along the main trunk. However in this case, the reason can be seen in the wet dark seepage at the base of the tree. This tree appears to be infected with some microorganism (Fungus? Bacteria?) which is excreting a smelly fluid out of a crack in the tree. When Ranger Deb and I bored into the tree, the heartwood was definitely wet and smelly, evidence that it was decaying. In these cases, the tree doesn’t do such a good job of controlling the sprouting of the buds on the tree trunk.
Finally this Douglas-fir near Old Main Trail, which still has many green needles, sports numerous fungal fruiting bodies which indicate it is heavily decayed.
Plants are persistent, but…
Sometimes the trees have problems from which they never recover. The red alder (Alnus rubra) pictured below probably just aged out. Estimates of what constitutes “old age” for an alder varies from 60 years to a maximum of 100 years. Red alder is a species that likes full sun light and most frequently gets started on disturbed sites. So no surprise that we would find one this size dead.
A little more surprising is the dead western redcedar (Thuja plicata) pictured below.
This species is very shade tolerant, and under normal circumstances commonly lives several hundred years. So why is this relatively young tree dead? My best guess is based on the fact that this was found on the uphill side of the trail. Trails often serve as unintentional “dams” to the normal flow of underground water (great example: Old Main Trail near the Nature Center). A couple of years ago we had an extraordinarily rain-soaked winter season and I hypothesize that this cedar got “drowned out.” Yes, cedar frequently grows in wet-ish areas, but there is a limit to everything.
Individuals from several shrub species have recently died as well. This red elderberry (Sambucus racemosa) located just off the Old Main Trail (pictured below) is the plant that began my awareness of this topic and thus this whole article. This plant died before the recent winter with heavy rains. It was the first plant which was part of my multi-year phenology study that died. Additional walks around the park revealed many other dead elderberries. Again, it appears to be a fairly short-lived plant.
Perhaps the most dramatic die-off I’ve witnessed occurred near the upper section of the Red Fox Trail. Last year I noticed that many of the Indian plums (Oemleria cerasiformis) seemed to turn yellow and lose their leaves a little earlier than normal. This year, a relatively large number of them never leafed out. I laid out a 1/20 acre plot (a circle with a radius of 26.3 feet) and counted all of the Indian plum stems. I also measured their diameters at ground level. To the best of my ability, if I was able to determine that multiple stems were part of a single plant, I only measured the largest stem. Important confession: I chose an area with a very high density of dead stems. The results are summarized below:
In some cases these plants were quite large, both in height and diameter. I laid one of the stems on the sidewalk near the top of the Red Fox Trail to make it easy to see.
I selected a few of the larger Indian plums, and counted the annual rings at the base of the stem. They were between 15 and 19 years old.
As a final example, I’ve also noticed this year a number of dead salmonberry (Rubus spectabilis) in the forest. I don’t think this is the result of some climatic fluke or disease, because there are also a very large number of healthy salmonberries in every area where I’ve see a dead one. One example of a dead salmonberry is pictured below:
Below is a cross-section of stem from a dead salmonberry. Note the relatively large whitish pith in the center.
And let’s not forget the animals
Sometimes animals play important roles in the life of plants. A couple of years ago, beavers decided that a lot of the young cedar trees near Obie’s Bridge were ready to eat, and went in for the harvest. The results were evident by the number of chewed off stumps, like the one seen below.
“This too shall pass”2
The forest we see today is not the forest we will see tomorrow. Barring huge environment shifts, the major trend that we should expect is that much of our uplands forest will evolve to a predominantly redcedar-hemlock forest type. Douglas-firs will be relegated to a tiny role. Red alders may persist in some of the bottomlands near the creek. This of course will bring some shifts in the animals that inhabit our forest as well. It will be different, but still just as fascinating as it is today!
1Documentation on the web indicates this phrase was used as early as 1409 during the installation of the Pope.
2According to Wikipedia, this is an ancient Persian expression that worked its way into the English language sometime in the 1800s.
By Bruce Rottink, Volunteer Nature Guide and Retired Research Forester
In the forest, like much of life, timing is everything! It’s why most animals have their young in the spring or early summer when food is abundant. It’s why most plants don’t bloom in December, when there’s a good chance that their flowers would be killed by a subsequent frost. The study of the timing of different biological events is called “phenology.”
What’s a Phenology Study?
A phenology study involves identifying when different organisms enter different stages of their life, or behave in particular ways. My phenology study focuses largely on plants. Plant phenology frequently involves studying the behavior of selected individuals over the course of several years. Some “events” in a plant’s life that can easily be tracked are, for example, when the veins on the new leaves are first visible (aka, bud break), or the first time you can see the sex organs inside of a flower.
I started the phenology study at Tryon Creek State Natural Area (TCSNA) early in 2013. For this study I tracked ten different species growing along 4 different trails at the park; Red Fox, Old Main, Cedar/West Horse Loop (referenced here as “Cedar”) and Middle Creek/Big Fir (referenced here as “Middle Creek”). For perennial species with above ground parts, I tagged the plants and followed them each year. For annual plants, or those species arising from underground organs, I identified a given patch of ground and studied plants at that location. As time went by, I started including observations on a few other species like a delightful patch of bleeding hearts (Dicentra formosa), and some spittlebug nymphs (suborder: Auchenorrhyncha).
I made observations on a weekly basis, with a few exceptions caused by vacations, and extreme weather conditions. For a couple of reasons, mostly related to “learning curve” issues, I starting collecting useable data in 2013 part way through the growing season.
One of the challenges in conducting a phenology study is the issue of when you should report the results. In this case, the differences I observed between 2016 and 2017 are dramatic enough that it is time to provide you with a report. This probably won’t be my last phenology report, “God willing and the creek don’t rise” (to use an old expression).
The Drivers of Plant Development
Plant development is driven by several factors, key among them being day-length, temperature and moisture availability. When it comes to spring budburst in our area of the world, temperatures probably are the primary driver.
The temperature plays two important, and quite different, roles in bud break. The plant needs to hold off on bud break until the threat of a killing frost is past. Thus most perennial plants in our area have a “chilling requirement.” This means that the buds have to experience a certain amount of chilling before they can start growing. Secondly, the buds have a “forcing requirement” which is a certain amount of warm temperatures to get the buds growing after the chilling requirement has been met. As anyone who has ever walked through the forest in the spring knows, these requirements vary dramatically between different species of plants. If the plants receive less than the normal amount of chilling curing the winter, they will need a greater amount of warm “forcing” in the spring. Although it is clear from the diagram that there is at least some minimal amount of chilling needed to ensure that the buds will eventually open.
The diagram below shows the generalized nature of the relationship of chilling and forcing for both Douglas-fir (Pseudotsuga menziesii) and western hemlock (Tsuga heterophylla). While the curves have a basically similar shape, it is apparent that with low levels of chilling, the hemlock will break bud first, but with large amounts of chilling, the Douglas-fir will break bud first.
Chilling and forcing requirements of Douglas-fir and western hemlock1
In reviewing these bud break results, please be aware that there is very little agreement on the exact temperature that separates the “chilling” and “forcing” functions. I believe most scientists would think 50 degrees is little bit too high, but to be blunt, this is the base I used because it is the best database to I have available. As you can see in the chart below, there have been dramatic differences between years in the number of growing degree days in the first three months of the year, primarily that 2017 has a much cooler spring.
*Data from the Aurora Airport, approximately 15 miles from TCSNA.
So What Happened?
Presenting even a summary of all the data I collected would be a sure cure for insomnia, so I’ve picked out a couple of examples from the study which are fairly typical of the general trends. The first example is to look at the behavior of the Pacific waterleaf (Hydrophyllum tenuipes). This is a plant that has underground roots, stems and buds.
The graph below shows the results for the years 2015 through 2017. On average, the appearance of the first leaves in spring 2017 was delayed an average of 2.5 weeks from the first appearance of leaves in the prior two years. (And yes, the absence of data for the week of Feb 11, 2015 is unfortunate!) The date of first flowering in 2017 was on average 3.0 weeks later than first flowering in 2016. The primary lesson here is that both budburst and flowering in 2017 was much delayed compared to the two prior years. Interestingly, the average date of when the last leaves died was nearly identical in 2016 (33.0 weeks) and 2017 (33.75 weeks).
Similarly, the leafing out of the vine maple (Acer circinatum) was also delayed about 3 weeks in 2017, as seen in the chart below. For the vine maple, so few of the plants I followed produced flowers on a regular basis that the data is probably not worth presenting, although what data there is follows the same general pattern as the Indian plum above.
At first glance, these lines all look fairly similar. However, looking at Week 20, for example, the number of growing degree days in 2016 is about double the number for that same date in 2017. This is a huge difference!
The final set of plant data that I will include here is for snowberry (Symphoricarpos albus), a reasonably common, but not abundant shrub at TCSNA. Here again, both the budburst and flowering of these plants is three to five weeks later in 2017 compared to 2016, as seen in the graphs below:
And it’s not just plants
Below is a chart of the sightings for two years of spittlebugs. In the beginning of 2016, if there were no spittlebugs seen, I just left the space on my datasheet blank. Midway through that year, I recognized the folly of that approach, and started making a clear record showing that no spittlebugs were seen. My notes on the March 2016 spittlebug indicate that it was just one individual bug, and a small one at that. Sometimes Mother Nature will show off one specimen of something way out of season, but that probably doesn’t really mean the season has started. No matter what you think about the March 2016 outlier, it is abundantly clear that the spittle bug, like many of its botanical associates, was late in 2017 by at least three weeks.
The Connection to Global Warming
As documented here in the differences between the various years, the organisms in the forest are sensitive to environmental temperatures. This will be very important as we consider global warming, and what we should do about it. I love the fact that Mother Nature provided us with a great example of the fact that the trend of global warming is not a straight linear process, but will have some hiccups like 2017. By the way, the early readings from this year (2018) show that some plants are starting growth way earlier than they did in 2017. But that’s grist for another note at least a year away. A fascinating aspect to this, which we may be many years away from experiencing, is that almost all woody plants require a certain amount of chilling before they break bud. If global warming ever gets to the point where the winter temperatures are not adequate to chill the buds, already completed research tells us bud break will be significantly delayed. Then we could have real problems. But for now, enjoy the forest that we have!
1Harrington, Connie and Peter Gould. 2016. Rise and Shine: How Do Northwest Trees Know
When Winter Is Over? Science Findings, Issue 183. USDA Pacific Northwest Research Station.